Top-Level Assembly and Mating Techniques

1. The Onshape Mating Philosophy: Connectors and Degrees of Freedom

Before we start clicking, it's important to understand how Onshape handles assemblies, as it's different from some older CAD systems. Instead of mating geometric faces and edges directly, Onshape uses Mate Connectors.

A Mate Connector is essentially a coordinate system (an origin with X, Y, and Z axes) that you can place on a part. A Mate then defines the relationship between two of these connectors, specifying which degrees of freedom (DoF) are allowed between them. As you know from kinematics, any rigid body in space has 6 DoF: 3 translational (movement along X, Y, Z) and 3 rotational (rotation about X, Y, Z). Every mate you apply works by constraining one or more of these DoFs.

This video gives an excellent conceptual breakdown of this philosophy.

2. Assembly Workflow: Building from a Fixed Base

A robust assembly starts with a solid foundation. The standard professional workflow is to bring in your main structural component first and lock it in place. All other parts are then mated relative to this fixed base.

Your Task: Start the Assembly

1. In your Onshape document, create a new Assembly tab.
2. Click the Insert button on the toolbar. Navigate to your Part Studios and insert the structural frame you created in the last lesson.
3. Click the green checkmark to place the frame at the assembly's origin.
4. Now, right-click the frame in the Instances list on the left and select Fix. You'll see an icon appear next to its name, indicating it's locked in space. This prevents the entire assembly from floating around.

3. Applying Mates: From Static to Dynamic

With the frame fixed, we can begin adding the other components and defining their relationships. We'll work through the main mate types needed for our project. The Onshape Help documentation provides a comprehensive reference for all mate types.

Let's see a practical demonstration of the basic assembly process.

Your Task: Assemble the Components

Follow these steps in your Onshape assembly. The hotkey for the Mate tool is M.

1. Mount the Guide Rails (Fastened Mate):

   • Insert your two linear guide rails.
   • Use the Fastened mate to attach them to the top surface of your frame. A Fastened mate removes all 6 DoF, rigidly connecting two parts.
   • Pro Tip: To mate the rail's bottom face to the frame's top face, you can select the center of each face's mate connector. If you want to align it by an edge, you can select mate connectors on the edges.

2. Add the Bearing Blocks (Slider Mate):

   • Insert two bearing blocks.
   • Use the Slider mate to attach one block to each rail. A Slider mate allows for one degree of freedom: translation along the Z-axis of the mate connector.
   • Select the mate connector in the center of the bore of the bearing block and the mate connector on the edge or face of the rail. Onshape is smart enough to create a sliding relationship along the rail's length.
   • After applying the mate, try dragging the blocks. They should slide freely along the rails but shouldn't be able to lift off or twist.

3. Install the Lead Screw (Revolute Mate):

   • Insert your lead screw, lead screw nut, and the motor/bearing mounting brackets.
   • Fasten the brackets to the frame.
   • The lead screw needs to be held by the bearings in its mount but be free to rotate. Use a Revolute mate between the shaft of the lead screw and the bore of the bearing/bracket that supports it. This allows only one DoF: rotation.

4. Simulating the Actuator: The Screw Relation

Now for the most important part: linking the screw's rotation to the linear motion of the nut. We do this with a Mate Relation.

First, we need a moving platform. Insert a simple plate (you can model a basic rectangular plate in a new Part Studio if you haven't already). Fasten this plate to the top of your two bearing blocks and Fasten the lead screw nut to the underside of this plate. Now, this entire platform should slide back and forth with the bearing blocks.

Next, we'll create the screw mechanism.

Your Task: Create the Screw Relation

1. Find the Screw relation icon in the assembly toolbar. It looks like a screw and is typically grouped with other mate relations.
2. The dialog will ask for two mates to relate:
   • Mate 1: Select the Revolute mate you created for the lead screw.
   • Mate 2: Select one of the Slider mates you created for the bearing blocks.
3. Distance per revolution: This is the critical value. Enter the lead of your lead screw. For example, if your screw is a TR8x8, the lead is 8 mm. Make sure your units are correct.
4. Click the green checkmark.

Now, test your assembly! Click and drag the lead screw to rotate it. The entire platform should slide smoothly along the guide rails. You've just created a fully functional virtual mechanism.

Conclusion

Fantastic work today! You have successfully transitioned from a folder of parts to a dynamic, functional assembly. This is the moment in the design process where you can truly validate the mechanical concept and see it in motion.

Key Takeaways:

• A robust assembly starts with a fixed base component to ground the design.
• Onshape's mate connectors and degrees-of-freedom approach provides a powerful way to define relationships between parts.
• Standard mates like Fastened, Revolute, and Slider handle the majority of assembly requirements.
• Mate Relations, like the Screw relation, are used to create complex kinematic links between different motions in the assembly.

Preview of the Next Lesson:
With a complete 3D assembly, the next logical step in a professional workflow is creating the documentation. In our next lesson, we will create an assembly drawing in Onshape and place standard orthographic and isometric views. We will also generate our first preliminary Bill of Materials (BOM), a critical document for manufacturing and procurement.